We used to walk on our knuckles

Human beings evolved from an ancestor that walked on its knuckles, just as the African apes (chimpanzees and gorillas) do today - conclude researchers in this week's issue of Nature.

A comparison of wristbones (Pic: B. Richmond)

Brian G. Richmond and David S. Strait of George Washington University, Washington DC reanalysed the wrist bones of early human fossils Australopithecus afarensis ('Lucy') and found they share the same knuckle-walking anatomy that chimps and gorillas have today.

Such early hominids - members of the human family - lived in Africa between 4.1 and 3 million years ago. Like humans, they walked fully upright. This suggests that a wrist anatomy suited for knuckle-walking was a relic of a still earlier stage in human evolution more than 5 million years ago, before the lineages leading to humans, chimps and gorillas became distinct.

Later hominids, such as Australopithecus africanus, had no trace of knuckle-walking adaptations and were fully bipedal, as are modern humans, Homo sapiens.

The new research suggests that the ancestors of humans did not immediately assume an upright stance after they climbed down from trees, but adopted a repertoire of movements including climbing and knuckle-walking. Why knuckle-walkers should then become bipedal is still a matter of debate.

Mark Collard and Leslie C. Aiello of University College, London, discuss the context and ramifications of this work in an accompanying News and Views article.

Richmond and Strait's study, they comment, is significant because it bears on the long standing debate over the evolutionary relationship between modern humans, chimpanzees and gorillas.

"Genetic analyses overwhelmingly indicate that chimpanzees and humans are more closely related to one another than either is to gorillas," they write. "Until the study by Richmond and Strait, however, the anatomical evidence largely ran counter to this conclusion."

They say that many palaeoanthropologists took the knuckle-walking characters of chimpanzees and gorillas as evidence they were more closely related.

"Richmond and Strait's results offer strong support for the idea that knuckle-walking characters were present in the common ancestor of modern humans and the African apes, and provides grounds for reconciliation between the molecular and anatomical evidence," write Collard and Aiello.